Fix leader-election RBAC, Go version drift, PVC churn, cron names, apply order

Addresses PR review + acceptance findings:

- Add leader-election RBAC: namespaced Role (coordination leases +
  events) and RoleBinding to the operator ServiceAccount, so controllers
  actually start under --leader-elect instead of looping on "leases
  forbidden". Verified in a kind cluster: lease acquired, both
  controllers start workers.
- Align Go versions: bump Dockerfile.operator to golang:1.26-alpine and
  CI images to golang:1.26 to match go.mod (go 1.26.5); set
  GOTOOLCHAIN=local so the image build stays hermetic (no toolchain
  download).
- PVC controller: add an annotation predicate so only PVCs carrying (or
  transitioning off of) backups.unkin.net/schedule enqueue, eliminating
  reconcile churn from unannotated PVCs while keeping the teardown path.
- Cron validator: accept alphabetic month/day-of-week names (MON, JAN,
  MON-FRI) that k8up and CNPG's robfig/cron accept, still rejecting
  unknown names; add unit tests for both mapping paths.
- Deploy ordering: extract the Namespace into its own manifest and add a
  config/kustomization.yaml so `kubectl apply -k config` creates the
  namespace first; document it in the README.
- Note the credential-less push precedent (jellyfin-ha) in docker.yaml.

Dockerfile.operator

@@ -1,4 +1,8 @@
-FROM golang:1.25-alpine AS builder
+FROM golang:1.26-alpine AS builder
+
+# Fail rather than download a toolchain: the base image must already satisfy the
+# go directive in go.mod so the build stays hermetic (no network at build time).
+ENV GOTOOLCHAIN=local
 
 WORKDIR /build